Protecting offshore structures from ice

ABSTRACT

This invention relates to a method of protecting a structure such as a vessel or platform located in water covered with sheet ice. In a preferred embodiment, an ice trenching machine is suspended from a boom supported at the top of the structure. The ice trenching machine removes ice in a circular path at a rate approximately equal to the rate of movement of the sheet ice toward the protected structure. Means are provided so that the boom can be moved entirely around the top of the structure so that a trench can be cut in the ice in any direction from the structure or to completely surround it.

United States Patent [1 1 Howard i451 Oct. 23, 1973 PROTECTING OFFSHORE STRUCTURES ,383,957 4/1959 114/ FROM ICE 2,449,963 9/1948 Ward [75] Inventor: George C. Howard, Tulsa, Okla. Primary ExamineFJacob Shapiro [73] Attorney-Paul F. Hawley et 'al.

Assignee: Amoco Production Company, Tulsa,

Okla.

ABSTRACT [22] Filed: Apr. 13, 1972 This invention relates to a method of protecting a Appl' 243319 structure such as a vessel or platform located in water covered with sheet ice. In a preferred embodiment, an ice trenching machine is suspended from a boom supported at the top of the structure. The ice trenching 114/42 E02b 15/02, B63b 35/12 [51] Int. machine removes ice in a circular path at a rate approximately equal to the rate of movement of the [58] Field of Search 61/1, 46.5, 46, 36 A,

, 41 42 sheet ice toward the protected structure. Means are rovided so that the boom can be moved entirely Referen e Cit d around the top of the structure so that a trench can be UNITED STATES PATENTS cut in the ice in any direction from the structure or to 9/1972 completely surround it.

u e Q 4 Claims, 2 Drawing Figures SHEETI C E MOVEMENT t BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a method of protecting a structure set in water which is coveredby sheet ice. It relates especially to those areas in which the sheet ice has a relatively slow movement; I

2. Setting of the Invention Some areas of the ocean are covered with sheet ice for a good part of the year. One such area is the Arctic. The sheets of ice normally vary in thickness from about I to feet. The sheet ice covers relatively hundreds of square miles. Any structure which is in the path of the movement of sheet ice is subjected to tremendous forces. Various means have been proposed to protect these structures. One such means is to build the structure sufficiently strong to withstand the maximum expected force. The main deficiency of thissolution is of course the high cost of such construction. It has also been suggested to build a protective icering around a vessel in such waters. (See' the article Polar Operations by Edwin A. McDonald, Captain USN (Retired) U.S. Navel Institute, Annapolis, Maryland, copyrighted 1969, of particular interest, pages 148-155.) In that method, water or crushed ice is continually added to make a thickened or strengthened section to protect the structure. There the author suggests separating the strengthened section fromthe outer ice pack only during its construction. In the present invention Im not concerned about building a strengthened section. I teach a way of protecting the structure by providing means for removing the -ice from the leading edge of the ice sheet as it approaches the structure at a rate approximately equal to the rate of movement of the sheet Ice.

BRIEF'SUMMARY OF THE INVENTION This invention relates to a method of protecting a structure such as a vessel or platform located in water covered with sheet ice. In a preferred embodiment an ice trenching machine is suspended from a'boom supported at the topof the structure. The ice trenching machine cuts completely through the sheet ice and removes ice at a rate approximately equal to the rate of movement of the sheet ice toward the protected structure. Means are provided so that the boom can be moved entirely around the structure on a selected path so that a trench can be cut in the ice on any side of the structure or completelyaround it.

BRIEF DESCRIPTION OF THEDRAWINGS DETAILED DESCRIPTION OF THE INVENTION Attention is directed to FIG. 1 which shows astructure 10 supported from thebottom of a body of water which is covered with sheet ice 12. Structure 10 has a top deck 14 which is typically supported 50-l00 feet above the sheet ice or water surface in the summer. Mounted on the deck 14 is crane 16 having a horizontally extending boom 18. A line extends from a power drum not shown on crane 16 over the endof boom 18 and drops down to an ice trenching machine desired area. Ice trenching machine 22 digs a trench 28 i on the side of structure 10 from which the sheet ice is moving. The trench 28 is preferably cut at a distance from about -100 feet from structure 14. The width of trench 28 is typically 5 to 10 feet.

As shown in FIG. 2 ice, trenching machine 22 has tracks 30 and 32 for propelling the trenching machine 'forwardi Element 34 represents the actual trench digging mechanism which can be any suitable means for breaking the ice and carrying the'chunks upwardly to conveyer belt 36 which conveys the ice away from the trench, 28 to a pile 38. This cutting blade 34 has a forward cutting edge or teeth 39and preferably also has aside cutting edge or teeth 41. Side teeth 41 are on the side away from structure 10 and would chip away at edge 29 of trench 28 during the faster movement of the .sheet ice toward the structure. Means are also provided for raising and lowering element 34. Such means are not shown in detail as any well trainedmechanic could build suitable means such as are provided, for example, on conventional ditch digging machines.

Most of the area where my invention is particularly suitable is along the coastal regions of the Arctic which has what is called stable ice sheet which is also termed landfast ice and is characterized by horizontal daily movement of the order of 10 feet or less. This relatively slow movement of the sheet ice toward structure, 10 gives me adequate time to continually remove ice with -my ice tjrenching machine at a rate approximately equal to the rate of movement of sheet ice 12 toward structure 10. In fact, if I cut trench 28 10 feet wide, it will ordinarily be a day before the ice 12 has moved sufficiently to close trench 28. To protect structure 10, all

I have to do is to keep removing ice with ice trenching machine 22 so that trench 28 never quite closes. If necessary,more than one trenching machine may be employed.

, While the abovedescription as been done wand tail, various modifications of the invention can be made without departingfroin the spirit or scope thereof.

least partially surrounding said structure on the side thereof toward'the movement of said ice sheet toward said structure, I

maintaining said trench in an open condition by removing ice at a rate approximately equal to the rate ported from said boom, there being no other connection between said trenching machine and said structure.

4. A method as defined in claim 2 in which said trench is cut at a distance of at least 50 feet from said structure. 

1. A method of protecting a structure in water supporting an ice sheet which has limited movement, which comprises: removing ice with an ice trenching machine by cutting a trench through the ice sheet in a path located a substantial distance from said structure and at least partially surrounding said structure on the side thereof toward the movement of said ice sheet toward said structure, maintaining said trench in an open condition by removing ice at a rate approximately equal to the rate of movement of sheet ice toward said structure so that said trench is never closed by incoming ice.
 2. A method as defined in claim 1 including the step of supporting said ice trenching machine from said structure only by non-rigid support means.
 3. A method as defined in claim 1 including the step of supportiNg a boom at the top of said structure and supporting said ice trenching machine with a cable supported from said boom, there being no other connection between said trenching machine and said structure.
 4. A method as defined in claim 2 in which said trench is cut at a distance of at least 50 feet from said structure. 